A system in which an air-fuel ratio sensor is positioned upstream of a three-way catalyst in an exhaust path for an internal combustion engine is disclosed by Japanese Patent Laid-Open No. 306742/1998. This system calculates an air-fuel ratio control parameter from the output of the air-fuel ratio sensor and uses the calculated parameter to adjust the air-fuel ratio for a target air-fuel ratio.
The output of the air-fuel ratio sensor may be in error because the output characteristic of the air-fuel ratio sensor varies from one unit to another and changes due to sensor aging. If there is an error in the output of the air-fuel ratio sensor on the upstream side in a situation where the control parameter is calculated only from the output of the air-fuel ratio sensor, the error directly affects the control parameter. As a result, air-fuel ratio control accuracy deteriorates.
In the above system, therefore, an exhaust gas sensor is positioned downstream of the three-way catalyst in addition to the air-fuel ratio sensor, which is positioned upstream of the three-way catalyst. This exhaust gas sensor is used to correct the output of the air-fuel ratio sensor on the upstream side. More specifically, a correction value for correcting the output of the air-fuel ratio sensor on the upstream side is calculated in accordance with the output of the sensor on the downstream side. The output of the air-fuel ratio sensor on the upstream side is corrected by the correction value. The control parameter is then calculated from the corrected sensor output. The above system curbs the influence of an error in the air-fuel ratio sensor on the upstream side because such a parameter is used for air-fuel ratio control.
[Patent Document 1]:
Japanese Patent Laid-Open No. 1998-306742
[Patent Document 2]:
Japanese Patent Laid-Open No. 1999-237361